Fix a check in the objc trampoline handler

Summary:
The function FunctionCaller::WriteFunctionArguments returns false on
errors, so they should check for the false return value.

Change by Walter Erquinigo <a20012251@gmail.com>

Reviewers: jingham, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D22278

llvm-svn: 275287